The new M4 is lighter than its predecessor and marks a return to the magically balanced and insanely quick BMWs we love and cherish

Originally published: June 12, 2014

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KOHLER, WISC. — Here is the most important thing you need to know about BMW’s new M3: the all-new 2015 M4 that Driving just finished testing is the first M3 that is lighter than the car it replaces. Oh, to be sure, the BMW salesman trying to sell you one of his little road rockets is going to wax lyrical about the return of the BMW’s iconic inline six, that the addition of not one, but two turbochargers produces monumental torque and the fact that, despite the loss of two pistons and 1,000 cubic centimetres of displacement, the new M4’s 3.0L has 11 more horsepower than the old M3’s V8.

Nonetheless, I can assure you that the biggest news in M-land this year is that the new M4 is 54 kilograms lighter than the old M3. That may seem a fairly picayune number, but for the M faithful, it at least signals a halt to the continuing cycle of weight gain that threatened to derail any semblance of sportiness from BMW’s sportiest cars. Indeed, what was once the most lithe of sports sedans — the original M3 weighed but 1,243 kilograms — had morphed, by 2013, into a 1,680 kilo porker. That is, for those who still struggle for context for any metric figure above two kilograms, about 960 pounds added weight. Had this weight gain kept apace, the 2015’s “improvements” might have seen the M4 break the 4,000-lb. (1,818 kg) threshold, turning what was once the world’s greatest sports coupe just another Gran Turismo.

Thank the Lord, then, that Albert Biermann, head of M Engineering, still believes that an M car still “has to perform as well on the track as it does on the commute.” So, not only has the 2015’s weight been reduced compared with the outgoing V8 model, Biermann notes that where that weight has been saved is equally significant. The two biggest factors in the M3’s Weight Watchers plan — the carbon fibre roof and the downsizing to six, rather than eight, cylinders, both pay dividends beyond just their absolute weight reduction. The loss of weight up high, thanks to that lighter roof, reduces the centre of gravity, always a good thing resulting in less unnerving body roll in fast corners and quicker transitions in tight left-right kinks.

The new S55 six-cylinder engine pays even bigger dividends. Not only does it place less weight over the front wheels, but it also reduces something called polar moment of inertia, which, in layman terms, is simply a measure of how enthusiastically — or reluctantly — a car turns from its previously straight path. Too much weight at either end slows things down; amass more of the weight in the middle or, better yet, reduce it overall and one has a lower polar moment of inertia, which, to the person behind the wheel, manifests itself as more of that enthusiasm for deviating from the straight and narrow.

The famous inline-six makes its return in the M4.Handout, BMW

And, despite the engine’s 38% greater torque, the new seven-speed DCT transmission’s quicker shifts, the power of the carbon ceramic brakes and even the M4’s incredible 40,000 Newton-metre chassis rigidity, it is the new car’s enthusiasm for deviating from said straight lines that is the biggest improvement over the outgoing M3. Fifty-four kilograms is not a dramatic weight savings and, indeed, more than a few new cars have lost that much avoirdupois without significant improvement, but the new M4, unlike virtually every M car of late (save the recent M235), drives smaller than it looks (always a good thing in a sports car). Turn-in is wonderfully precise (despite the change to oft-maligned electric power steering), the suspension takes a set without delay and the balance between front and rear end grip — something even heavy M cars have always had — is as magical as always. Those who feared that BMW was losing its focus — Yours Truly among them — can breathe easier: The newest M4 may mark a reversal of the GT’ing of M cars.

Indeed, the only damper on the M4’s handling sweetness is electronic; compared with more liberal vehicle stability controls systems — those by Chevrolet and Porsche, for instance — the DSC traction nanny halts the party just as the Rolling Stones start playing the first note of It’s Only Rock and Roll. Such is the prowess and stability around a racetrack that it deserves something like Corvette’s Competition Mode so that the M4 can let its hair down and slide, if not quite shake, its booty around.

2015 BMW M4Handout, BMW

Lighter, of course, also makes the engine’s job easier, though in the case of the M4’s twin-turbo 3.0L, it hardly needs it. According to Biermann, the company’s trademark inline six is particularly well suited to the plumbing that is part and parcel of turbocharging. Further, he says the new low-mass turbochargers (that spin to an incredible 200,000 rpm) are particularly friction-free, meaning that turbo lag — that dreaded delay between driver’s command and the engine’s response — is minimized. Indeed, what most warms Biermann’s cockles about the S55 is not being able to brag about its 425 horsepower or even its 406 pound-feet of torque, but that the latter is produced at a subterranean 1,850 rpm while the former holds on to 7,300 rpm (it may not be unusual for a turbocharged engine to produce max torque below two grand, but it is very unusual for that same engine to be then able to rev to 7,300 rpm and beyond). It’s a motor powerful beyond its displacement and civilized beyond its power, exactly what you’d expect from a powerplant massaged by M.

It’s only flaw is that those two torque-producing turbochargers also muffle what is often described — even if it’s only me doing the describing — the most mellifluous engine this side of a Ferrari V12. Turbochargers get their power-boosting impetus from the energy wasted in exhaust gasses. That may do wonders for efficiency and power production, but it takes some of the hard-edged rasp out of an engine that, back in the day of 3.0L M3s and 3.6L M5s, made American V8s sound positively tame. Ironically, all that turbocharger interference with exhaust pipe pressure waves has the inline six sounding like a V8 despite its inline orientation and 120-degree crankshaft.

2015 BMW M4Handout, BMW

There’s a bunch more technology that goes to making the M4 so much more than the sum its parts. Indeed, it’s a travesty not to mention the electronic-controlled Active M rear differential whose ability to transfer power to the appropriate rear tire is so much a reason that BMW could afford to liberalize its vehicle stability control system. But, assuming that BMW is right about who buys M4s (the young or at least young at heart) and what they do with them (i.e. frequently “track” them), then the other big news, technology-wise, is the new GoPro app that allows you to control your “hero” camera remotely through iDrive. Link a new GoPro’s wi-fi hookup to your cellphone and the ConnectedDrive app allows you to start/end recording as well as change resolution and framing, all without the fiddling normally associated with GoPro-ing.

The assumption, again, is that your M4 is going to spend enough time braking, accelerating and slewing sideways on a racetrack to warrant the automation. Without BMW stopping M’s slide into GT mediocrity, however, all this four-wheeled narcissism would all have been wasted.

Editor’s note: For those of you not au courant with all things BMW, though we are testing what would nominally be seen as an all-new model line — the M4 — it is simply what enthusiasts have long known as the M3 Coupe. What used to be known as the M3 — available, depending on the model year, as either coupe or sedan — has morphed, thanks to some marketing maven’s brilliant strategic plan, into the M3 sedan and the M4 coupe. So, for the sake of clarity when we are making comparisons between the old and new versions — the 2013 M3 and the 2015 M4 — we are, in fact, comparing like models, despite their differing last digit.